The invention relates to a process for the purification of polyalkylene glycol dialkyl ether solutions contaminated with one or more glycols such as triethylene glycol.
In a number of chemical industries, mixtures of polyalkylene glycol dialkyl esters (hereinafter designated PG ethers) are conventionally used for absorbing acid gases, such as hydrogen sulfide and carbon dioxide, from gas mixtures such as natural gas, refinery gas, synthetic gas, ammonia synthesis gas, and the like. U.S. Pat. No. 3,737,392 to Ameen and Furbush discloses the treatment of a gas mixture by employing a recirculating solution of PG ethers to absorb acid gases from the gas mixture being treated. The absorbed acid gases are then removed from the PG ether solution by flashing.
A problem with the use of PG ether solutions is that they can become contaminated with chemical compounds which adversely effect their use. For example, glycol compounds from an upstream glycol dehydration process can contaminate PG ethers used in a downstream gas treating facility. Glycol dehydration is frequently used upstream of a natural gas treating facility to remove water from the natural gas by extracting the water therefrom with a glycol solvent. Glycol dehydration typically occurs prior to passing the natural gas through a pipeline to the gas treating facility, in order to prevent pipeline corrosion. Additionally, slugs of glycol may be periodically injected into the pipeline to further control corrosion. In the course of the dehydration operation, it has been found that some of the glycol is vaporized into the natural gas. Subsequently, in the gas treating facility, the glycol in the natural gas can be absorbed by a recirculating PG ether solution, used to treat the gas. Additionally, glycol may be carried into the PG ether solution by entrainment. In any case, the amount of glycol in the recirculating PG ether solution gradually builds up and may reach equilibrium concentrations as high as 35 percent.
The presence of a glycol contaminant in the PG ether solution has an adverse effect on its acid gas absorption capacity for which it is used in the gas treating facility. Additional recirculation is then required to achieve the same gas clean-up. This additional recirculation increases the operating expense of the gas clean-up, as well as decreasing the gas capacity of the gas treating facility. An alternate means to reduce the glycol contamination is to replace the PG ether solution with fresh solution. However, this is normally unacceptable due to the high cost of the PG ether solution.
It has been proposed to remove the glycol contaminant from the PG ether solution by distillation. Unfortunately, such a distillation would be undesirable due to the low pressures and high temperatures that would be required. Atmospheric conditions cannot be employed due to thermal stability limits on the PG ether solution. Additionally, such a distillation would entail a glycol purge that would involve substantial losses of the PG ether solution, especially desirable lower molecular weight components.
Several patents disclose the removal of various contaminants from a PG ether solution. For example, U.S. Pat. No. 4,334,102 to Decker et al discloses a method for removing liquid hydrocarbons. Decker et al utilizes an aqueous salt solution which is mixed with the contaminated PG ether solution in a tank. The aqueous phase is heated and separated into two phases in a vessel. The purified PG dimethyl ether solution is withdrawn from the vessel.
U.S. Pat. No. 3,831,348 to pap discloses a process to separate sulfur bearing compounds from PG ethers solutions. Pap utilizes low amounts of a water immiscible liquid hydrocarbon extracting solvent to achieve the desired separation.
The prior art processes for removing contaminants from a PG ether solution are not directly applicable when a glycol is the contaminant. Thus, the need exists for a process of removing glycol contaminants from a PG ether solution.